1. Technical Field of the Invention
The present invention relates to a diaphragm inserted with an optical system, such as an imaging lens into a lens barrel and put to use.
2. Description of the Related Art
A diaphragm is incorporated in a lens unit of a camera and the like. A variable diaphragm unit a diameter of an aperture of which can be regulated in accordance with the brightness and the like is often used as a diaphragm incorporated in a lens unit. The lens units incorporated in, for example, a simplified photographic camera like a lens-carrying film unit, and the lens units of a simplified digital camera incorporated in a portable telephone also include a lens unit not mounted with a variable diaphragm. The light unnecessary for a photographing operation enters a lens unit in no small quantities to cause a flare and a ghost to occur, and the nobility of a photographic image to lower in some cases. For this reason, a fixed diaphragm having a constant diameter of a diaphragm aperture, and capable of preventing the incidence of harmful light and setting a F-value of a lens is used for the lens unit for the above-mentioned simplified camera.
A diaphragm formed by kneading carbon black and the like in a plastic sheet so as to give the light shielding property thereto, and providing the resultant plastic sheet with a diaphragm aperture has heretofore been utilized as such a fixed diaphragm. However, in order to maintain the strength and light shielding property of the plastic sheet, it is necessary that the sheet has a thickness of at least around 0.2 to 0.3 mm. However, a photographic lens used for, for example, a portable telephone has been miniaturized to the extent that a total length of the lens is reduced to around several millimeters. A further reduction of the thickness of the diaphragm as a whole has also been demanded with consideration given to a space in which the diaphragm is to be inserted. In the diaphragm made of one piece of the above-mentioned plastic sheet, the thickness thereof cannot be reduced satisfactorily since the diaphragm has the problems of the strength and light shielding property thereof. Even when a sheet of around 0.2 to 0.3 mm in thickness is used, a ghost/a flare occurring due to the reflected light from the edge portion of the diaphragm necessarily causes the deterioration of the property of the diaphragm.
The diaphragm known from JP-A-11-212138 (hereafter “JPA '138”) is formed by laminating a thin sheet provided with a diaphragm aperture of a predetermined diameter therein on a thick sheet provided with an aperture of a diameter slightly larger than that of an object aperture. Since the diaphragm aperture is formed in the thin sheet, the unnecessary reflection of the light on an inner circumferential surface of the diaphragm is reduced, and the strength and light shielding property of the diaphragm is maintained owing to the thick sheet.
However, the diaphragm disclosed in JPA '138 needs to prepare two pieces of sheets formed by the press working respectively, and an operation for pasting the sheets on each other accurately with a bonding agent so that the centers of the diaphragm aperture are aligned with each other becomes necessary, so that the manufacturing cost increases. Since the thin sheet provided with the diaphragm aperture is as thin as 0.04 to 0.09 mm, the surrounding portion of the aperture, which is not laminated on the thick sheet, of the thin sheet has light transmission of around 5%, i.e., the light shielding property was unsatisfactory.